Electron emitting device and method of making



June 30, 1931.

D. M RAE ELECTRON EMITTING DEVICE AND METHOD OF MAKING Filed Sep i922INVENTOR DUNCAN MAcRnE Patented June 30,1931

UNITED STATES.

PATENT OFFICE DUNCAN MACBAE, OF EAST ORANGE, NEW JERSEY, ASSIGNOB TOWESTINGHOUSE LAMP COMPANY, A CORPORATION OF PENNSYLVANIA.

ELECTRON EMITTING DEVICE AND METHOD OF Application filed September 11,1922. Serial No. 587,270.

This invention relates to electron-emitting devices such as radio-bulbs,X-ray tubes and the like and more particularly to compositeelectron-emitting cathodes therefor and methods of making the same.

An object of my invention is the provision of a simple and eflicientmethod of making oxide-coated electron-emitting filaments adapted foruse as cathodes in electron-tubes and the like.

Another object of my invention is the production of filament or ribbonwith a firmly-adherent, stable coating of a compound thereon which isadapted to be converted to an oxide or mixture of oxides with highelectron-emission characteristics.

A further object of my invention is the manufacture of oxide-coated,electron-emitting cathodes without the use of an organic 2o binder orthe like.

A still further object of my invention is the manufacture ofplatinum-iridium filament or ribbon coated with a stable, alkalineearthcarbonate-or mixture of alkaline-earth carbonates, which are adapted tobe decomposed when placed in an electron device, leaving a firmlyadherent coatingof an electronemitting oxide or mixture of oxides onthefilamen Other objects and advantages of this invention will be madeapparent upon reading the following description.

It is well known that when filamentary'material is coated with certainoxides, the electron-emissivity of such material is greatly increased.Several methods have been proposed for applying such an oxide-coating tofilamentary material. One of such methods,

consisting in coating platinum-iridium rib- 40 bon with a mixture ofbarium and strontium carbonates by means of an organic binder andafterwards applyin heat to convert such carbonates to oxides, isescribed in the paper by H. D. Arnold, entitled Phenomena inoxide-coated filament electron tubes and published in the PhysicalReview, vol. 16, page 73, (1920). I

Another method as proposed consists in dipping a platinum-iridiumfilament a large number of times into solutions of nitrates of themetals, the oxides of which it is desired to apply and heating suchdipped filament to convert the nitrates to oxides after each dipping.The use of organic binding material is undesirable, as it has to beeliminated eventually and the use of onl soluble salts, such asnitrates, entails an un ue amount of labor,

because of the large number of dippings required.

According to my invention, I appl a coating of carbonates of thealkaline-cart metals by dipping once or a few times only, suchcarbonates being readily converted into oxides by heating in anelectron-device, without introducing useless substances other thancarbon dioxide, into the bulb or evacuated vessel thereof. Briefly, mymethod may consist in forming an inorganic suspension of alkalineearthcarbonates, preferably the carbonates of barium and strontium in wateror other suitable liquid, inert toward the carbonates, and which isreadily removed, by heating for a short while, without leaving anappreciable organic residue, passing filamentary ma.- terial to becoated continuously through the suspension and then through heatingmeans for drying and baking the same to causeit to adhere to thefilament. means may comprise an electric tube-furnace, which preferablycontains an atmosphere of carbon dioxide to prevent undue decompositionof the carbonates. v

This process may be repeated a number of times until the required weightof coating is applied to the filament. When using certain filaments, forexample, those having very smooth surfaces, it may be desirable to cleanthe filament by first passing it through means,

suchasan electric tube furnace, for burning oil from the surface anyimpurities before passing it through the carbonate suspension. A morefirmly adherent coating of such carbonates may be secured by theaddition of a small proportion of a chloride or nitrate, for

example, barium chloride or barium nitrate,

such substances acting as an inorganic binder, but ordinarily not beingnecessary.

A' filament prepared according to the aforedescribed method will keepindefinitely in the air and the carbonate thereon may be actifrom thepump or exhaust machine. In the a A. first instance, it is activatedduring exhaust for example, the wider the ribbon, if such is by passingelectric current through the filament or ribbon. In the second, it isnecessary to provide a clean-up agent, such as aluminum or magnesium,for removing the carbon dioxide resulting in the decomposition of thecarbonate coating on the filament.

My invention will better be understood by referring to the accompanyingdrawing, in which the single figure illustrates apparatus by means ofwhich filamentary material may be coated with carbonates according to myinvention.

A filament 1 may be wound from spool 2 over or around a pulley orgrooved wheel 3, for applying a coating thereto of a carbonatesuspension 4 which may be held in a cup 5. The cup 5 with the wheel 3therein'may be an ordinary gettering cup such as is generally used inthe manufacture of vacuum tungsten lamps, but any suitable means may beused for uniformly coating the filamentary material 1 with the desiredsuspension 4.

The filament 1 may be platinum-iridium ribbon, nichrome which is anickel chronium alloy or any other desired fairly refractory filamentarymaterial. The size and shape of the wire or ribbon determines, to someextent, the concentration of the carbonate suspension,

used, the thinner or more dilute maybe the suspension applied thereto.,The coating is preferably applied by passing the wire a plurality oftimes through the suspension and heating between each application,although said coating may be applied in one operation, if desired.

As an example of how the suspension may be prepared according to thesize of wire or ribbon used, the following details are given,

but I do not intend tolimit myself to said details, as the same are notessential, but merely illustrative.

When using 3 mil round wire, a fairly thick aqueous batch or suspensionis preferable and the following proportions of ingredients may be used;9 grams of pure powdered barium carbonate maybe mixed with 10 grams ofpure powdered strontium carbonate, preferably in a mortar, and all lumpspulverized and ground up therein with a pestle. 18 c. c. of water maythen be added, a little at a time, first forming a thick paste bystirring the mixture up with the pestle and then gradually thinning outsaid paste by adding the rest of the water. By this means, a suspensionof the carbonates of barium and strontium is prepared, in which thesolid ingredients are finely divided and not subject to rapid settling.

This suspension is preferably quickly poured into the cup 5 and thewheel 3 rotated by any suitable means. Rotation of the wheel 3 serves tokeep the suspension stirred up and to prevent the solid ingredients fromsettling out. Wl1en using a. fairly wide ribbon, for example, aplatinum-iridium ribbon 10 mils wide and .3 of a mil thick, a thinner ormore dilute suspension is permissible, for example, 35 c. c. of watermay be used for the same weight of carbonates, instead of 18 c. 0. Sucha suspension is used and the coating is applied by passing said ribbonthrough the suspension a plurality of times and drying and baking aftereach coating. If, however, it is desired to apply the coating in oneoperation, the suspension may be made more concentrated by decreasingthe amount of water used. For example, if only 18 c. c. of water areused instead of 35, good results are attainable by passing such a ribbonthrough the suspension only once and drying the same thereon.

When using a fairly narrow ribbon, however, for example, one about 2mils wide and 4 mil thick, a suspension of about the same consistency asthat used on the 3 mil round wire is preferably employed, that is, thesmaller and narrower the wire or ribbon, the more concentrated shouldthe suspension be, although it is not desirable to use less water than18 c. c. with the amount of carbonate heretofore mentioned.

If the filamentary material to be coated is so smooth in surface thatthe pure carbonate will not adhere firmly enough thereto, a smallproportion, for example, from about to about 2 grams of barium chlorideor the same amount of barium nitrate may be added to the suspension.Such an addition increases the fluidity of the carbonate suspension,making it permissible in some cases to use less water, and causes thecarbonate coating, when heated, to adhere more firmly and coherentlythan when the sameis pure. As an alternative, a carbonate suspensionmaybe made using a nitrate solution, for example, in any concentration, asthe liquid vehicle. That is, the addition of other substances to thecarbonates, or the utilization of less-pure carbonates, will cause thecoating of the same to adhere more firmly to the filamentary material.Barium chloride has an additional effect upon the coating, in that itcauses the same to bake on firmly in an almost transparent film.

The filament 1, after emerging from the suspension 4, passes throughmeans for heating the same to dry and bake the coating thereto. anelectric tube furnace 6, about 13" long and about 1" internal diameter,energized from any suitable source of electricty (not shown) so that abright red heat is obtained upon the interior of the furnace. Thefilamentary material is preferably passed through the furnace at amoderate rate of speed, for ex- S'uch means preferably comprises ample,16 to 18 meters per minute, and the interior of the furnace preferablycontainsan atmosphere of carbon dioxide, for example, a stream of carbondioxide may be allowed to flow through the same from .a reservoir 7 orother source thereof through the valve 8, tube 9 and from there to oneend ofthe furnace so that it escapes from the other end. After emergingfrom the furnace, the filament 1 may be wound up on a spool 11 which maybe rotated by any suitable driv' ing means (not shown) for winding upthe filament.

The spool 11 and the filament wound thereon may then be transposed withthe spool 2 and the operation repeated as many times as desired, or, ifthe concentration of the sus pension is sufficient to attain the desiredresult with only one dip, the filament will then be ready for use in anelectron device or the like.. 7

It has been found desirable, especially with platinum-iridiumfilamentary matenal, to pass such material through the tube furnace toburn off impurities thereon before passing the same through thecarbonate suspension. In order that all of the filamentary material tobe coated may receive such coating prior to its passage through the tubefurnace, the same may be threaded through such furnace by first passinga wire or the like of any other desired material throu h filament to becoated t erethrough by means of such other wire. V

After baking the carbonate coating on the filamentary material, the sameis ready to be introduced into an electron device, or, the filament maybe first activated by heating in an inert environment to a temperaturehigh enough to change the carbonates to'oxides. If introduced into anelectron device while in the form of'carbonates, the filament may beheated to decompose the carbonates while the device is still on thepump, in which instance, the pump will remove the liberated carbondioxide. If, on the other hand, the

filament is not heated until after the device is sealed off, a suitableclean-up agent, for example, aluminum or magnesium or both must it anddrawing the prising applying thereto a mixture of the carbonates ofbarium and strontium and barium chloride and heating the filament tocause the same to adhere firmly thereto.

4. An electrode adapted to be rendered thermionically active comprisingan electrically conductive body having a coating of alkaline earthcarbonates containing a small proportion of a soluble alkaline earthsalt baked thereon.

5. an electrode adapted to be rendered thermionically active comprisinga platinum alloy wire having a mixture of the carbonates of barium andstrontium and a small proportion of barium chloride applied thereto andbaked thereon so that the coating is firmly adherent.

In testimony whereof, I have hereunto subscribed my name this 8th day ofSeptember,

DUNCAN MACRAE.

be provided to eliminate or clean-up the liberated carbon dioxide.

The clean-up agent, in the form of a plate or foil, is preferablyattached to the anode or plate of the electron device and vaporized toactivate the same, by heating said plate by high-frequency induction orelectron bombardment, after the device has been evacuated I and sealedoff, whereby residual and occluded gases are eliminated.

Although I have described what I now consider to be the preferred meansfor practicing my invention, it is to be understood that manymodifications may be made therein and that I desire to be limited onlyby the spirit and. scope of the appended claims.

